The invention disclosed herein pertains to electrothermally actuated switches which are sensitive to variations in current flow for providing overload protection to electrical apparatus. More specifically, the invention relates to overload relays wherein elements constructed of shape memory affect alloys are employed as thermally responsive switch actuating elements.
Overload relays are utilized in circuit with electrical apparatus such as motors or the like to disconnect electric power from the apparatus in the event current flow to the apparatus increases to a level which could potentially damage the apparatus being controlled. U.S. Pat. No. 3,478,292 issued Nov. 11, 1969 to U. F. Carter et al, discloses one form of commercially available overload relay wherein heater coils are disposed around latching, or detent, assemblies which are filled with a eutectic alloy solder. The heater coils are connected in circuit with the controlled electrical load device. Under acceptable current levels the eutectic alloy solder is in a solid form to latch a rotatable shaft rotation by a spring loaded switch actuating mechanism. When current flowing to the controlled device exceeds a predetermined level, it generates heat within the heater coil to cause the eutectic alloy solder to melt, thereby releasing the rotatable shaft and permitting the spring loaded system to drive an operating shaft for actuating switch contacts. The arrangement of the switch contacts and the spring forces of the contacts and operating springs must be carefully determined and selected to achieve the desired operation of the switch contacts. To enable this overload relay to be utilized with load devices having different current ratings, the heater coils must be removed and replaced with similar coils having different thermal outputs.
A similar overload relay is shown in U.S. Pat. No. 3,566,328 issued Feb. 23, 1971 to U. F. Carter et al, wherein the eutectic alloy solder-filled latch structure is replaced by a stacked arrangement of frusto-conical bimetal disks which expand in response to heat generated by the heater coil to drive an actuator mechanism for actuating an overcenter switch contact structure. A limited adjustment to the thermal response of the bimetal disks is provided in this device to calibrate the response of the disks to the tolerances of the particular heater elements utilized. However, the heater coils must be exchanged for coils having different thermal outputs when utilizing this overload relay with a load device of a different current rating.
Another form of commercially available overload relay is disclosed in U.S. Pat. No. 3,800,260 issued Mar. 26, 1974 to F. N. Woodger. In this device a plurality of cantilever supported bimetal members are individually wrapped with heater coils such that the bimetal elements will warp in a given direction in response to heat generated by current passing through the heater coils. The bimetal elements work upon a lever and slide system to operate a snap-action switch contact structure upon a predetermined movement of the lever and slide system. An adjustment mechanism is provided in this device which operates upon the lever and slide system to afford a selective range of currents at which the switch contacts will be actuated. While these devices are satisfactory for their intended purposes, the overload relay of this invention provides improvements thereover.